Have you said... acid?

@Rob691,thank you so much for your suggestion, i'm trying to do it and i think the plants are responding pretty well!

@Cheapgrowing, i had the same problem. My fault was to control PH before adding nutrients in the water. The soil went down to 5.5 at the girls start starving ! I have solve the issue now by feed plants with water AND nutrient at 7Ph for a couple of weeks.

@RipperSeeds, hi there! Yes, i've always controlled the pH solution and noticed the deficiency beginning a couple of weeks ago and since the last week i've started to make it higher. Before that happend it was always around 6, now I'm starting to use the solution going from 6.5 to 7 to see what happens